The article by D. Lanfranchi et al., “A Microprocessor-Based Analog Wristwatch Chip with 3 Seconds/Year Accuracy”, IEEE 1994, describes a circuit making it possible to increase the stability of the nominal frequency of a quartz-type resonator typically having a frequency equal to 32 kHz. An oscillator is connected to this resonator. A dual-frequency operating mode of the resonator is provided: the frequency adjustment is done by proceeding with a switching of a capacity added to the structure of the oscillator, by an external reference signal. Thus the oscillator, qualified as dual-frequency, can oscillate at two different frequencies: a first frequency greater than the nominal frequency and a second frequency lower than the nominal frequency. A ratio can be defined between the respective average times that the oscillator spends at the two high and low frequencies, corresponding to the first higher frequency and the second lower frequency, respectively. This ratio is adjusted using a reference signal. This reference signal can derive either from a more precise second resonator or from a temperature sensor. The more precise resonator can be used intermittently. However, it consumes too much to play the role of a time base serving to maintain a real time clock (RTC). Furthermore, the use of two resonators is not compatible with a concern for maximum miniaturization of the electronic devices.
It may therefore be preferable to obtain the time base by integrating a silicon-type low frequency resonator, combined with a temperature sensor. For example, document JP 58 173488 discloses a thermal compensating device for a dual-frequency oscillator, implementing a counter allowing a periodic modification of the frequency of the resonator as a function of the measured temperature, and therefore a temperature compensation of the output frequency.
However, a so-called dual-frequency mode device only makes it possible to compensate the variations of the resonator's frequency as a function of the temperature on a scale corresponding to the so-called draw difference between the high frequency and the low frequency. The variation of the frequency of a silicon resonator, as a function of the temperature, is close to 30 parts per million (ppm) per degree Celsius. It has been shown that one could obtain a draw in the vicinity of 100 ppm for a resonator of this type. The silicon resonator can then only be compensated over a range of about 3 degrees Celsius, which is insufficient for an industrial application.
In document EP 1 475 885, a thermal compensation similar to that proposed by document JP 58 173488 is done but, furthermore, a variable frequency divider is placed on the outlet of the oscillator, the measured temperature acting, via a compensating circuit, on a bank of switchable capacities controlled digitally (or on a variable capacity controlled by a digital-analog converter) and also on the division factor of the divider. Thus, the temperature compensation range is extended. However, the variable frequency divider is realized using a PLL, the N/M factor of which is close to the unit, which penalizes the consumption. Moreover, due to the non-linearity of the adjustment features, a calibration must be done at several different temperature points, making the device complex to implement.
The present invention proposes a temperature compensating device for a resonator making it possible to avoid these drawbacks.